In the related art, as an optical recording medium of an optical information recording and reproduction device, an optical disk such a CD for audio use or a DVD for video use are commercially available. In recent and continuing years, development is made on high quality DVDs having even higher capacity, for example, a DVD using a laser beam of a short wavelength, like a blue laser (blue laser DVD). These optical disks are used not only as read-only media, but also as external storage devices of video recorders, personal computers, etc., and are practically applied in optical information recording and reproduction devices capable of writing and reading data in the optical disks.
FIG. 9 is a diagram illustrating an example of a configuration of a portion of an optical pickup of an optical information recording and reproduction device (optical disk drive) for writing and reading data in the optical disk.
In FIG. 9, first, from a light source 11 using a semiconductor laser (LD), a linearly polarized light beam is emitted, and after being converted into a parallel light beam by a collimator lens 12, the linearly polarized light beam is incident on a polarized beam splitter (PBS) 13. Depending on the polarization direction of the incident light beam, the polarized beam splitter 13 reflects the incident beam or allows the incident beam to transmit. The polarized beam splitter 13 is provided to constitute an incident path of the linearly polarized light beam emitted from the light source 11 toward an optical disk, and a returning path of the light reflected from the optical disk toward a light detector.
After transmitting through the polarized beam splitter 13, the linearly polarized light beam emitted from the light source 11 is reflected by a launching mirror 14 in a direction to the optical disk, and transmits through a ¼ wavelength plate 15. At this moment, the linearly polarized light beam is given a phase difference equaling ¼ wavelength and becomes a circularly polarized light beam, transmits through an electro-optical element 16, and is condensed on the optical disk 18 by an object lens 17, thereby allowing recording data therein or reading data of pits therefrom.
The light reflected from the optical disk 18 becomes a circularly polarized light beam rotating in a direction opposite to that before the reflection. The circularly polarized light beam rotating in the opposite direction on the returning path transmits through the electro-optical element 16, and passes through the ¼ wavelength plate 15 again, where the linearly polarized light beam is given a phase difference equaling ¼ wavelength again, and becomes a linearly polarized light beam having a polarization direction different from that of the light beam on the incident path by 90°. As a result, the light beam on the returning path is incident into the polarized beam splitter 13 again, and is reflected by the polarized beam splitter 13, in contrast to transmission of the incident light beam on the incident path, is condensed by a detection lens 19, and is incident on a detector 20. The detector 20, for example, includes plural divisional light receiving elements, and outputs various servo signals, like focus servo or track servo, or a detection signal used for generating information reproduction signals.
In an information recording and reproduction device with sufficient efficiency being obtained in the incident path to secure adequate recording power, a R disk (dye) or a RW (phase change) disk can be recorded using the maximum power of the LD. The optical system used in such a situation may be structured by the polarized light optical system using the above polarized beam splitter 13, so that at the same time, sufficiently high efficiency is obtainable in the returning path during detection.
If the optical pickup having the configuration shown in FIG. 9 is adopted, when using the above-mentioned optical disks having high capacity, such as DVDs, or blue laser DVDs, a problem to be solved is variation of optimal recording conditions caused by the optical path difference of the optical disk. The optical path difference of the optical disk is due to the optical residual strain possessed by the transparent substrate fabricated by molding a poly-carbonate resin. Hence, before being reflected on the optical disk 18 and returned, the circularly polarized light beam passing through the ¼ wavelength plate 15 is affected by the phase difference of the optical disk 18, and becomes an elliptically polarized light beam. Afterwards, the light beam incident on the polarized beam splitter 13 is split into a polarized component, which is reflected and serves as a detected light beam, and a polarized component returning to the illumination optical system including the LD. Under this condition, generally, the LD noise increases because of the light returning to the LD, and the signals of the light reflected from the optical disk 18 are degraded because of variation of the LD emission power, which depends on a recording waveform.
Further, fluctuation of the optical path difference in the radial direction of the optical disk 18 arises when the temperature inside the device goes up or when the optical residual strain of the disk substrate varies; this fluctuation induces degradation of splitting capability of the polarized beam splitter 13 regarding the beam to the detector 20, and induces significant reduction of the detection signal. This fluctuation further causes it to be difficult to perform operations with high precision, such as focus servo or track servo, trial writing for deducing the optimal recording power to be used in recording, or recording power control during recording.
This problem occurs especially in an optical information recording and reproduction device for DVDs or the like, using a red laser. As disclosed in Japanese Laid Open Patent Application No. 2000-268398, to reduce the influence of the aforesaid optical path difference of the optical disk 18, as shown in FIG. 9, the electro-optical element 16, formed from a liquid crystal element, is arranged in the optical path of the optical pickup. After transmitting through the electro-optical element 16 formed from liquid crystal elements, the incident light beam is condensed on the optical disk 18 for recording and reproduction. When the reflected light passes though the electro-optical element 16 again, which is formed from liquid crystal elements, by controlling alignment of the liquid crystal molecules in the liquid crystal layer, a phase difference is given to the liquid crystal molecules such that the phase difference of the optical disk 18 is cancelled out. Due to this, the incident light beam is completely reflected on the polarized beam splitter 13, and the reflected light is directed to the detector 20, thereby preventing reduction of the detected light.
Further, an optical path difference detector 21 is provided, and a generator, such as an amplitude detection circuit, is used to generate a signal indicating the phase difference of the optical disk 18, while maintaining a constant angle between a polarization direction of a light beam, perpendicular to the optical axis of the light beam, and the alignment direction of the liquid crystal elements, the alignment direction of the liquid crystal elements being controlled such that the angle between the optical axis and the alignment direction of the liquid crystal elements in a plane including the optical axis changes to induce cancellation of the phase difference of the optical disk 18 in the light beam passing through the liquid crystal elements.
Although the phase difference of the optical disk 18 can be cancelled out by using the electro-optical element 16, the structure of the optical pickup becomes complicated, and the transmittance of the light beam in the incident path and transmittance in the returning path decline. Further, the cost of the device increases.
Further, the aforesaid elliptically polarized light beam component generated due to the optical path difference of the optical disk cannot be split by the polarized beam splitter 13 or a polarized beam hologram, and this reduces the detected light beam.
Concerning such an optical path difference of the optical disk, the elliptically polarized light beam component of the light beam from the LD light source can be separated into a P-polarized light component and an S-polarized light component by a separate polarized beam splitter, and light amounts of the components can be measured by separate light receiving elements. Further, without using the light beam from the LD light source, it is also possible to detect the amounts of light corresponding to the optical path difference of the optical disk by arranging separate LD light sources and light receiving elements besides optical systems for recording or reproducing the optical disk.
However, arrangement of electro-optical elements for correcting the optical path difference of the optical disk suffers from limitations in optical layout of the optical pickup device, a rise of cost due to an increased number of parts, the shape of the optical pickup, or an increased number of signal lines, and other problems.